IRAQI GYPSEOUS SOIL STABILIZED BY ORDINARY AND ENCASED STONE COLUMNS

Several model tests were performed to improve gypseous soil from Iraq with ordinary (common) and encased stone columns under static loading. The model tests performed on untreated soil were taken as a reference to the proposed improvement techniques, for which the stress ratio (
/) values equal 0.75.The mode of failure seems to be local shear failure type which gradually changes towards the general shear when using ordinary and encased stone columns. For ordinary stone column (OSC), the stress ratio(
/) increases to 1.17 at failure with increasing settlement ratio due to stiffness of stone column compared to untreated (normal) soil. The bearing improvement ratio (
/
)shows a peak value at failure (S/B=0.537), then drops down and remains nearly constant with increasing S/B ratio. This difference in behavior may be attributed to the gradual changes in the mode of failure. The settlement reduction ratio decreases to 0.48 at failure with increasing stress ratio(
/). It is found that there is slight increase in bearing capacity to 1.25 for soil treated with encased stone column (ESC) compared to soil treated with ordinary stone column. Also, the bearing improvement ratio (
/
) shows a slight increase to about 1.66 at failure. Finally, the settlement reduction ratio ( / )at failure was 0.5 without any significant effect observed between ordinary and encased stone columns. Ke